As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas- and light-barrier material, e.g. an aluminium foil or an ethyl vinyl alcohol (EVOH) foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. The web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which may be fed to a folding unit to form the finished packages.
More specifically, the pillow packs substantially comprise a main portion, and opposite top and bottom end portions tapering from the main portions towards respective top and bottom sealing bands which extend substantially orthogonal to the axis of the pack. In detail, each end portion is defined by a pair of respective trapezoidal walls which extend between main portion of the pack and the relative sealing band.
Each pillow pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin projecting from respective sealing bands; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by respective trapezoidal walls.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing band.
Packaging machines for producing packages of the above type are known, substantially comprising:                an in-feed conveyor;        a folding unit receiving the pillow packs from the in-feed conveyor and adapted to fold these pillow packs to form relative parallelepiped-shaped packages; and        an out-feed conveyor which receives folded packages from the folding unit and moves them away from the packaging machine.        
Folding units are known, for example from EP-B-0887261 in the name of the same Applicant, which typically comprise:                an endless conveyor for feeding packs continuously along a forming path from a supply station to an output station;        a number of folding devices arranged in fixed positions relative to the forming path and cooperating with packs to perform relative folding operations thereon;        a heat-sealing device acting on respective triangular flaps of each pack to be folded, to melt the external layer of the packaging material and seal the flaps onto respective walls of the pack; and        a pressing device cooperating with each pack to hold the triangular portions on respective walls as these portions cool.        
In detail, the conveyor comprises an endless chain looped about and meshing with a driving sprocket and an idler wheel and formed by a plurality of links mutually connected by hinge pins at respective hinge points; the conveyor also comprises a tightener acting on the chain to maintain it at a constant tension.
The chain comprises a top straight branch, a bottom straight branch and two curved portions which are opposite to each other, respectively cooperate with the driving sprocket and the idler wheel and connect, on respective opposite sides, the top and bottom branches.
Though efficient, folding units of the above type leave room for improvement.
In particular, as the hingedly joined chain links are rigid, the chain substantially forms a polygon about the driving sprocket and the idler wheel. As a consequence, the radius of the chain varies periodically around the driving sprocket and the idler wheel; as the driving sprocket and the idler wheel rotate at a constant angular speed, the varying radius causes the linear speed of the chain to fluctuate and the chain links to rise and fall with respect to their line of engagement with the driving sprocket and the idler wheel. This latter movement of the chain links does not actually occur as it is compensated by the tightener. The above-described phenomenon is known as “polygon effect” and is more evident in chains having big pitches and meshing with sprockets having reduced numbers of teeth.
The continuous intervention of the tightener to maintain the chain at a constant tension produces a periodic vibrating motion, which may affect the packs being conveyed and the quality of the forming operations performed on the packs as they advance.